This PDF file contains the front matter associated with SPIE Proceedings Volume 8111, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

Up- and down-conversion (UC, DC) constitute two singular routes to achieve improved energy harvesting of sunlight by changing its shape of the solar spectrum. To obtain a significant conversion rate two main challenges have to be overcome: i) the excited lanthanide ions have to emit efficiently, a target which has been better accomplished for DC materials; ii) the absorption in the lanthanide-based UC and DC layers has to be high to ensure a sizeable fraction of photons can be harvested. In this paper, we review such materials and their use as spectral converters for photovoltaics (PV), paying special attention to the UC and DC processes in lanthanide glasses in fluoride matrices. We discuss the challenges that need to be overcome in order to implement these materials in real PV devices. Finally, we will present the synthesis of erbium (Er³⁺) doped YF₃ nano-crystals embedded in transparent glass ceramic (TGC) by melt quenching. This material presents a low phonon energy environment for the Er³⁺ ions due to the fluoride crystals, while the silica glass provides chemical and mechanical stability to the compound.

Novel metallic nanopatterns integrated with semiconductor films form optical metamaterials that can enable enhanced absorption. When employed in photovoltaics, such integrated nanostructures may facilitate significant increases in power conversion efficiency. Here, we show that metal nanopatterns embedded within a semiconductor, as opposed to being situated at/on the surface(s), exhibit absorbance enhancements that exceed those yielded by surface patterns. We show in computer simulations and experiments that absorbance in amorphous silicon can be enhanced many-fold for embedded metal nanopatterns (EMN) in ultrathin silicon films in particular wavelength regimes. Compared to plane α-Si films of comparable thickness, a several hundred percent enhancement is observed. This represents a potential route to high efficiency solar power with ultrathin absorbers enhanced by EMN-based optical metamedia.

Downconversion is investigated as a promising way to enhance silicon solar cells efficiency. The efficiency of the downconversion process is investigated for the (Pr³⁺, Yb³⁺) codoping in two fluoride hosts: KY₃F₁₀ and CaF₂. Strong near-infrared emission from ytterbium ions after excitation of praseodymium ions at 440 nm is observed in both KY₃F₁₀ and CaF₂ as a result of the efficient energy transfer from praseodymium to ytterbium. In particular, very high Pr³⁺ to Yb³⁺ energy transfer efficiencies (ETE) are achieved for low Yb³⁺ and Pr³⁺ concentrations (ETE=97% in CaF₂:0.5%Pr³⁺- 1%Yb³⁺) in CaF₂ in comparison with KY₃F₁₀. A low Yb³⁺ concentration offers the advantage to limit the Yb³⁺ concentration quenching which is observed in other hosts, where the Yb³⁺ concentration has to be larger to achieve a high ETE for solar cell applications.

Coatings of conducting gold-black nano-structures on commercial thin-film amorphous-silicon solar cells enhance the short-circuit current by 20% over a broad spectrum from 400 to 800 nm wavelength. The efficiency, i.e. the ratio of the maximum electrical output power to the incident solar power, is found to increase 7% for initial un-optimized coatings. Metal blacks are produced cheaply and quickly in a low-vacuum process requiring no lithographic patterning. The inherently broad particle-size distribution is responsible for the broad spectrum enhancement in comparison to what has been reported for mono-disperse lithographically deposited or self-assembled metal nano-particles. Photoemission electron microscopy reveals the spatial-spectral distribution of hot-spots for plasmon resonances, where scattering of normally-incident solar flux into the plane increases the effective optical path in the thin film to enhance light harvesting. Efficiency enhancement is correlated with percent coverage and particle size distribution, which are determined from histogram and wavelet analysis of scanning electron microscopy images. Electrodynamic simulations reveal how the gold-black particles scatter the radiation and locally enhance the field strength.

Light-trapping is a key issue for high efficiency thin-film silicon solar cells. In this work, the interaction of incident light with microcrystalline silicon solar cells applying a plasmonic reflection grating back contact is studied with threedimensional electromagnetic simulations and via the measured spectral response of prototypes. The investigated plasmonic reflection grating back contact consists of half-ellipsoidal silver nanostructures arranged in square lattice at the back contact of a n-i-p substrate type microcrystalline silicon solar cell. Experimental results of prototypes of these solar cells show significantly enhanced short circuit current densities in comparison to flat cells and even a small enhancement of the short circuit current density in comparison to the conventional random texture light-trapping concept of thin-film silicon solar cell. A very good agreement was found for the simulated and measured spectral response of the solar cell. From the simulated three-dimensional electromagnetic field distributions detailed absorption profiles were calculated allowing a spatially resolved evaluation of parasitic losses inside the n-i-p type microcrystalline silicon solar cell.

One way to successfully enhance light harvesting of excitonic solar cells is the integration of optical elements that increase the photon path length in the light absorbing layer. Device architectures which incorporate structural order in form of one- or three-dimensional refractive index lattices can lead to the localization of light in specific parts of the spectrum, while retaining the cell's transparency in others. Herein, we present two routes for the integration of photonic crystals (PCs) into dye-sensitized solar cells (DSCs). In both cases, the self-assembly of soft matter plays a key role in the fabrication process of the TiO₂ electrode. One approach relies on a combination of colloidal self-assembly and the self-assembly of block copolymers, resulting in a double layer dye-sensitized solar cell with increased light absorption from the 3D PC element. An alternative route is based on the fact that the refractive index of the mesoporous layer can be finely tuned by the interplay between block copolymer self-assembly and hydrolytic TiO₂ sol-gel chemistry. Alternating deposition of high and low refractive index layers enables the integration of a 1D PC into a DSC.

We investigated the effects of doping on the photovoltaic efficiency in a GaAs reference cell, and in undoped, n-doped, and p-doped InAs/GaAs quantum-dot (QD) solar cells. We found that the photovoltaic efficiency of the undoped QD solar cell is almost the same as that of the reference cell. However, the efficiency improves monotonically with increasing inter-dot ndoping, while p-doping deteriorates the photovoltaic conversion. We observed a 50 % increase in photovoltaic efficiency in the device n-doped to provide approximately six electrons per dot as compared with the undoped QD cell. In this QD solar cell, the short circuit current density increases to 24.30 mA/cm² compared with 15.07 mA/cm² in the undoped QD solar cell without deterioration of the open circuit voltage. To identify the physical mechanisms that provide this improvement, we investigated the spectral characteristics of the photovoltaic response and photoluminescence of our QD solar cells. We found that the electron capture into QDs is substantially faster than the hole capture, which leads to an accumulation of electrons in QDs. The electrons trapped in dots enhance IR transitions. The built-in-dot electron charge together with charged dopants outside the dots creates potential barriers, which suppress the fast electron capture processes and at the larger scale form a potential profile which precludes degradation of the open circuit voltage. All of these factors lead to the enhanced harvesting of IR energy and a radical improvement of the QD solar cell efficiency. Higher efficiencies are anticipated with further increase of doping level and at higher radiation intensity. This makes the QD solar cells promising candidates for use with concentrators of solar radiation.

Photon absorption, and thus current generation, is hindered in conventional thin-film solar cell designs, including quantum well structures, by the limited path length of incident light passing vertically through the device structure. Optical scattering into lateral waveguide structures provides a physical mechanism to dramatically increase photocurrent generation through in-plane light trapping. However, the insertion of wells of high refractive index material with lower energy gap into the device structure often results in lower voltage operation, and hence lower photovoltaic power conversion efficiency. In this work, we demonstrate that the voltage output of an InGaAs quantum well waveguide photovoltaic device can be increased by employing a novel III-V material structure with an extended wide band gap emitter heterojunction. Analysis of the light IV characteristics from small area test devices reveals that nonradiative recombination components of the underlying dark diode current have been reduced, exposing the limiting radiative recombination component and providing a pathway for realizing solar-electric conversion efficiency of 30% or more in single-junction cells.

Luminescent solar concentrators (LSC) concentrate both diffuse and direct radiation with no need for tracking. They consist of transparent plates doped with luminescent materials that absorb incident light. Most of the emitted light is trapped inside the plate by total internal reflection, where it is guided to solar cells at the plate's edge faces. We investigate the concept of a photonic LSC (PLSC) that mitigates the major LSC loss mechanisms, namely the escape cone and reabsorption of emitted light. Embedding the luminescent material in a photonic crystal allows highly efficient light guiding and can reduce reabsorption through inhibited emission at unwanted wavelengths. We present FDTD simulations that show how the emission characteristic is influenced by the surrounding structure due to an altered photon density of states. Further, enhanced light guiding in a broad spectral range was obtained with efficiencies of up to 99.7%. We also report on our progress in fabrication of PLSC devices for experimental investigation of the concept: polymer thin films with and without luminescent doping were spin coated and characterized to estimate the number of dye layers needed in PLSCs to achieve sufficiently high absorption.

We propose a new architecture of metal-insulator-metal devices for solar energy harvesting at infrared and visible frequencies based on asymmetrical alignment of insulating barrier relative to the Fermi level of metals and spatial localization of hot electrons excited by photons. Photons absorbed by metals create hot electrons, which can transmit through the thin insulating barrier, producing current. We theoretically investigated the photocurrent response and power generations at different wavelengths. Short circuit current and open circuit voltage can be easily tuned by changing metal thicknesses to adjust the forward and reverse photocurrent. By employing surface plasmons, power generation efficiency is enhanced 9 times in a grating MIM device compared to direct illumination at 650 nm. Finally, we compared the enhancement of power generation efficiency by SPs excited through grating structure and Kretschmann coupling system.

Core-shell silicon nanowire (SiNW) solar cells with an a-Si heterojunction were prepared on SiNW arrays, which were etched into n-type silicon wafers or into n-doped multicrystalline silicon thin films on glass substrates. A stack of intrinsic and p-doped hydrogenated a-Si was deposited as a shell around the SiNWs by PECVD, acting as a heteroemitter of the solar cells. Finally a TCO layer consisting of aluminum doped zinc oxide was deposited on top of the a-Si by atomic layer deposition. In a mesa-structured solar cell (area 7 mm2) an open circuit voltage of 476 mV and an efficiency of 7.3% were achieved under AM 1.5 illumination. Electron beam induced current measurements show clear evidence that most of the photo-current comes from the thin SiNW layer.

We study the optical response of various nanojunction solar cell architectures and examine how various cylindrical arrangements of emitter, base, glass and transparent conductor affect reflection and absorption of incident light. The photogeneration profiles of such nano-architectures are cylindrically asymmetric, varying axially, radially and azimuthally within the wavelength band investigated. Such 3D profiles require 3D device models for accurate device analysis. The extended nanojunction configuration was examined in more detail, as this design is known to have superior performance. The particular design consists of nanostructured glass and a superstrate arrangement of the other device elements.

In this proceeding, we use optical modeling and detailed balance analysis to predict the limiting efficiency of nanostructured silicon solar cells based on vertically-aligned nanowire and nanohole arrays. We first use the scattering matrix method to study broadband optical absorption. By incorporating the calculated optical absorption into a detailed balance analysis, we obtain the limiting short circuit current, open circuit voltage, and power conversion efficiency of nanowire and nanohole solar cells. Results show that optimized nanowire and nanohole arrays of 2.33 microns in height have 83% and 97% higher power conversion efficiencies than a thin film with the same height, respectively. Furthermore, we find that the limiting power conversion efficiency is mainly determined by the short circuit current density, which is proportional to the broadband optical absorption.

We have demonstrated a new type of hybrid solar cell based on a heterojunction between poly(3,4- ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) and vertically aligned n-type GaAs nanowire (NW) arrays. The GaAs NW arrays are directly fabricated by the nano-etching of GaAs wafer with spun-on SiO₂ nanospheres as the etching mask through inductively coupled plasma reactive ion etching (ICP-RIE) system. Then we attach GaAs NW arrays onto PEDOT:PSS conductive polymer to form a p-n junction. According to our research, the morphology of GaAs NW arrays strongly influences the characteristics of the GaAs NW/PEDOT:PSS hybrid solar cells. The improved interpenetrating heterojunction interface and the suppressed reflectance of GaAs NW arrays will offer great improvements in efficiency relative to a conventional planar cell. The power conversion efficiency of 5.8 % of GaAs NW/PEDOT:PSS cells under AM 1.5 global one sun illumination can be achieved.

Conventional manufacturing processes of solar cells, including epitaxy, diffusion, deposition and dry etching, are high cost and high power consumption. To save energy and reduce expenses, we use organic material, silicon nanostructure and solution process. The devices structure is n-type bulk Si (n-Si)/n-type silicon nanowires (n- SiNWs)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) heterostructure. The active region includes n-Si and n-SiNW arrays, promising the property of ultra low reflection for high light absorption. In this work, SiNWs of only a-few hundred nanometers could lower the reflectance to below 5%. In addition, an organic material - PEDOT:PSS, instead of p-type doping, is introduced to form a p-n junction with n-Si/n-SiNWs for separating the electron-hole pairs. The use of PEDOT:PSS can also passivate the surface defects of n-SiNWs. N-type SiNW arrays are made by aqueous etching process. The etchant contains Ag⁺ and HF etching vertically to the 1-10 Ω-cm Si (100) wafers. After etching and removing residual Ag and SiO₂ by nitric acid and diluted HF successively, n-SiNW arrays existed on either surfaces of n-Si with very dark color; then Ti and Ag were evaporated on n-Si to be a cathode. Finally, nanowires of n-Si/n-SiNWs were stuck on the PEDOT:PSS that were spin-coated on the ITO coated glass to form a core-sheath heterojunction. The performance and quantum efficiencies (QE) were measured. The short circuit current density and power conversion efficiency are 27.46 mA/cm² and 8.05%, respectively, which are higher than other solar cells containing SiNWs. The external and internal QE are beyond 50% and 60% in visible range, respectively.

To reduce cost and maintain viability of silicon-based solar cells for commercial applications, the absorption of thin silicon films, on the order of a few microns, must be enhanced. We are developing a method to increase absorption across the solar spectrum by exploiting the enhanced electric near field caused by surface modes of excited metallic particles. Previous research in the field has focused on deposition of these particles on a passivated Si surface, a technique that scatters light in directions lateral to the cell, but does not promote direct carrier generation, as the near field enhancement is too far from the Si itself. Consequently, we have developed a method using ion implantation and thermal evaporation to fabricate Ag nanoparticles below the Si surface, near the p-n junction, where the enhanced near field can greatly enhance the absorptive response of the silicon solar cell.

Silicon wire arrays have been synthesized through a two-step metal-assisted electrode-less etching from an n-type silicon wafer with (100) orientation. Field Emission Scanning Electron microscope (FESEM), Ultra violet-Visible-Near infrared (UV-VIS-NIR) spectrophotometer and Resonance-coupled photoconductivity decay (RCPCD) have been used to characterize the morphological, optical, and electrical properties of Si wires at varying etching times. The reflectivity of the wire arrays decreased with increasing etching time because of light scattering from the micro-roughness of the Silicon wire surfaces. The effective carrier lifetime decreased with increasing wire length due to the increased surface area. We also created smoother wire surfaces by thermal oxidation followed by HF dipping. From FESEM cross sectional images and reflectivity results, this treatment removes the micro-roughness, but the effective lifetime is lower than the as-grown wire arrays. A photoluminescence peak observed only in the smoother wires suggests that the lower effective lifetime is due to the diffusion of residual Ag atoms from the wire surfaces into the bulk during the thermal oxidation process.

Photovoltaic research has moved from popular solar cells, based on crystalline silicon substrates with thicknesses of around 250 μm, to the thin film structures saving large amount of the active material. The next generation of solar cells requires substantial increase of the energy conversion efficiency, which can be achieved by enhancing of the optical trapping inside the cell. In this work we study the efficiency of light trapping inside vertical silicon nanowire solar cells. The main focus is on the optical trapping inside single vertical nanowires, which can enhance optical absorption far beyond capabilities of a thin film. Spectral optical absorption modeling based on RCWA together with the electromagnetic field distribution analysis gave insight into the light trapping inside the nanowires. Results provide a guide for the optimization of nanowires diameters, density and length for maximal short circuit currents with minimal material demands.

We study solar-cell designs using nano-grating on both top (transmission) and bottom (reflection) of the solar cell. First, we perform simulations based on rigorous coupled wave analysis (RCWA) to evaluate the diffraction top gratins. In RCWA method, we calculate up to 20 harmonics, and sweep the launch angle of incident light from 0 to 90 degree. The incident light varies from100nm to 1200nm wavelength. Triangular grating can achieve higher light absorption compared to the rectangular grating. The best top grating is around 200nm grating period, 100nm grating height, and 50% filling factor, which responses to 37% improvement for triangular grating and 23% for rectangular grating compared to non-grating case. Then, we use Finite-Difference Time-Domain (FDTD) to simulate transmission/reflection double grating cases. We simulated triangular-triangular (top-bottom) grating cases and triangular-rectangular (top-bottom) grating case. We realize solar cell efficiency improvement about 42.4%. For the triangular-triangular (top-bottom) grating case, the 20% efficiency improvement is achieved. Finally, we present weighted-light simulation for the double grating for the first time and show the best grating can achieve 104% light improvement, which is quite different from traditional non-weighted calculation.

Creation of molecular asymmetry in the organic sensitizing dyes has been demonstrated for enhancing the photoconversion efficiency due to unidirectional flow of electron after the photoexcitation. Molecular structures for direct indole ring carboxy-functionalized unsymmetrical squaraine dyes have been optimized by fine tuning the molecular structures and judicious selection of the substituents to prevent the dye aggregation and electron recombination. Best efficiency of 4.42 % was achieved for unsymmetrical squaraine dye SQ-64 with a short circuit current density of 11.22 mA/cm2, a fill factor of 0.61 and an open circuit voltage of 0.64 V under standard AM 1.5 simulated solar irradiation.

At present, the light conversion efficiencies achievable with organic photovoltaic (OPV) technology are significantly below those seen in inorganic materials. The efficiency of OPV devices is limited by material properties; the high energy and narrow-band absorption of organic semiconductors results in inefficient harvesting of solar radiation, while the low charge carrier mobility in organic semiconductors limits the possible active layer thickness. Utilization of plasmonic structures in or around the OPV active layer has been suggested as a way to achieve a higher conversion efficiency in thin film photovoltaic devices. Our theoretical and experimental results indicate that aluminum-based plasmonic nanostructures hold significant promise for conversion efficiency enhancement in OPV devices. The high plasma frequency of aluminum permits a nanoparticle concentration close to the percolation threshold, which results in a broader band of plasmonically enhanced absorbance in OPV material and better overlap between the natural absorption bands of OPV materials and the plasmonic band of the metal nanostructure than what is achievable with gold or silver plasmonic structures. This is demonstrated experimentally by embedding aluminum nanoparticles in P3HT:PCBM layers, which leads to a significantly enhanced absorption over a broad range of wavelengths. While aluminum nanoparticles are prone to oxidation, our results also indicate the path to stabilization of these particles via proper surface functionalization.

Improvement of energy conversion efficiency of solar cells has led to innovative approaches, in particular the introduction of nanopillar photovoltaics [1]. Previous work on nanopillar Si photovoltaic has shown broadband reduction in optical reflection and enhancement of absorption [2]. Radial or axial PN junctions [3, 4] have been of high interest for improved photovoltaic devices. However, with the PN junction incorporated as part of the pillar, the discreteness of individual pillar requires additional conductive layer that would electrically short the top of each pillar for efficient carrier extraction. The fragile structure of the surface pillars would also require a protection layer for possible mechanical scratch to prevent pillars from breaking. Any additional layer that is applied, either for electrical contact or for mechanical properties may introduce additional recombination sites and also reduce the actual light absorption by the photovoltaic material. In this paper, nanopore Si photovoltaics that not only provides the advantages but also addresses the challenges of nanopillers is demonstrated. PN junction substrate of 250 nm thick N-type polycrystalline Si on P-type Si wafer is prepared. The nanopore structure is formed by using anodized aluminum oxide (AAO) as an etching mask against deep reactive ionic etching (DRIE). The device consists of semi-ordered pores of ~70 nm diameter.

The SiO_x/Si quantum wells (QWs) structures were fabricated by using the successive deposition technique, as quantum confinement device to increase the effective energy bandgap and passivation effect in Si surface for the third generation solar cell applications. In Si/SiO_x QWs, the thicknesses of Si layers and SiOx layers were varied between 1 to 5 nm, respectively. The roughness of sputter-deposited Si on SiO_x was less than 4 Å in the thickness of 2 nm. By using the SiO_x/Si QW structures on Si surfaces, the lifetime measured by u-PCD technique increased as a result of passivated surface effects. The tunneling phenomena and good interface properties were observed in the fabricated QWs structures.